1. Technical Field
The present invention relates generally to a method for measuring submicron images which are currently not measurable or resolvable by any known prior art technique.
As semiconductor features continue to shrink and the subtleties of pattern densities become apparent (e.g. photoresist thickness variations across a chip), the ability to determine the quality of the photoresist features, especially for vias and contacts, becomes increasingly difficult. Is the image open at the bottom? What is the size of the image at the bottom? These are the important questions which are answered by the present invention. As semiconductor images shrink to the sub-micron regime, the ability to determine masking image quality becomes increasingly difficult.
2. Prior Art
Photoresist patterns on semiconductor wafers are typically about 1.0 micron thick. As horizontal images drop below 0.5 microns, this creates a submicron aspect ratio of 2:1 or higher. An important factor in the success of transferring the photoresist image into the semiconductor lies in the ability to determine the image size at the interface between the photoresist and the semiconductor (the "bottom" of the photoresist image).
Currently, low voltage scanning electron microscopes (SEMS) are used to measure these images. Unfortunately, submicron high aspect ratio images in the photoresist pattern create structures which do not allow repeatable accurate measurements using this technique.
Standard practice includes running a wafer through an etch step and a resist removal step, and then measuring the resulting etched image. Downsides of this procedure include 1) the time lost while the wafer is processed, 2) if the images are out of specification the wafer must be discarded, and 3) it does not accurately determine if the photoresist image is completely open.
An alternative method is disclosed in Tiro-Lira et al. U.S. Pat. No. 5,493,116 wherein an SEM is modified to include two detectors which work in conjunction to provide an improved depth of field using backscatter emission. Although this technique may work with 0.5 micron images in 2 micron thick resist (as described in the patent), as images reduce even further this technique may soon become too limited.